Introduction
The main feature that must be considered when choosing the cable is the ability to handle the high current at LV level. Moreover, the cable must meet KAHARAMAA requirements for LV level either single core or multicore cables. The derating factors must be considered when choosing the cable as the cables are going to be installed underground. Since Doha cable are the main cable producer in Qatar, all the derating factor calculation must be done on Doha cable products to get the most suitable cable for distribution. Afterward, the derating current that been obtained will be compared with load current to select the appropriate cable. Finally, this process will be performed twice, first time for the cables between the transformer and LV panel, the other one is for the cables between LV panel and distribution boards.
- Multi-core cables constrain:
- core stranded copper containing a three-phase conductor and one neutral.
- The core must be XLPE insulated and each core must be coloured.
- The armour has to be galvanised steel wire.
- PVC over sheath.
- Single core cables constrain:
- Circular core of stranded copper.
- The core must be XLPE insulated.
- The cable the armour must be armour.
- PVC over sheath.
Connected Load Current
In this section, a calculation will be performed to compute the amount of the current that will be needed for each part of distribution as from TX until LV panel and from LV panel until the distribution board. This calculation is crucial because the amount of the current will decide which cable to be selected. Hence, all the calculations are performed at full load which can obtain from table 3-15. Equation (6) illustrate the total amount of current for each load either from TX to LV panel or from LV panel until distribution board:
I: The amount of current drowns Load (A) S: Required apparent power for Load (VA) VL-L: The base voltage of LV panel (V)
- Calculating the current from TX until LV panel
The following manipulation will be for the stadium, car parking and street LV panel:
Since, there are no suitable cables for such a current from Doha Cable and for the difficulties that could be faced such as binding the cable when the cable is installed due to it is large diameter, the current will be divided to two. Hence, two cables will be required for each phase.
- Calculating the current from LV panel until Distribution board
This calculation is for firefighting load for the sport activity from LV panel:
A similar calculation will be done for each load that goes from LV panel until its distribution board. The table 4-1 summarizes all the current loads for both transformers until LV panel and then from LV panel until distribution board for the load.
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Connected Load Current |
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|
TX until LV panel |
Current (A) |
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|
Stadium LV panel |
868.12 |
|
|
Sport activity LV panel |
991.23 |
|
|
LV panel until distribution board |
||
|
Sport activity LV panel |
AC |
488.65 |
|
Socket + pump |
248 |
|
|
Lighting |
458.3 |
|
|
Firefighting |
541.6 |
|
|
Stadium LV panel |
AC + change room |
100.35 |
|
Audience +bathroom |
239.25 |
|
|
Poles (1+2) |
484.54 |
|
|
Poles (3+4) |
484.54 |
|
|
Parking + streets |
379.4 |
|
|
Firefighting |
294.83 |
|
Derating Factor Calculation
In general, derating factor is a factor that will minimize the rating current of the cable. There are several types for the derating factors such as ground temperature, underground depth and solid thermal resistivity. In order to calculate the total derating factor of a system, all the various derating factor have to be multiplied together as equation (5) shows.
According to KAHARAMAA regulation, the temperature of Ground Temperature = 35 °C, the thermal resistivity quarry scalping = 150°C.cm/Watt. The depth of the cable should be 700mm [20].
Ground Temperature derating factor at 35 ℃ for XPLE cables=1
Soil thermal resistivity derating factor at 150 ℃ .cm/Watt=0.91
Derating Current of the Cable = Ground Temperature derating factor x Depth derating factor x Soil Thermal Resistivity derating factor x Rating current of the cable
Calculation
The cable with cross section 25 mm2 is going to be used in the project, therefore, to calculate it is derating factor the equation (5) will be used:
Derating Current of the cable = 1 x 0.98 x 0.8 x 130 = 102 A
The cable 800 mm2 will be installed in trench, therefore air temperature derating factor will be considered.
All the under cable 800 mm2 will be installed in ground, therefore ground temperature derating factor will be considered.
Since Doha cable do not have a product that can maintain the firefighting current in sport activity, it will be supplied by two cables.
The following table 4-2 summarize all cables derating current for selected cables that will be used in this project. While table 4-3, summarizes the specifications for cables required for every load in the sport complex. All the products specification and derating factor are in APPINDEX [C]
Summary
| Cables | ||||||||||
| Cable Cross Section (mm2) | 25 | 95 | 120 | 150 | 185 | 240 | 400 | 500 | 800 | |
| Rating Current (A) | 130 | 265 | 305 | 335 | 375 | 435 | 547 | 605 | 1292 | |
| Ground Temperature derating factor | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | ||
| Air Temperature derating factor | 1.05 | |||||||||
| Soil thermal resistivity derating factor | 0.91 | 0.91 | 0.91 | 0.91 | 0.91 | 0.91 | 0.91 | 0.91 | ||
| Depth Derating Factor | at 700 mm | 0.98 | 0.97 | 0.97 | 0.97 | 0.97 | 0.97 | 0.955 | 0.955 | 0.955 |
| Derating Current (A) | 116 | 234 | 269 | 296 | 331 | 384 | 475 | 534 | 1296 | |
Select the Cable
The selection of the cable is considered as crucial factor when designing the distribution network. If the cable, choose is underrating of the load current that cause the cable to heat up and could be melt and as results the power will be cut off from the user. In other hand, if the cable was overrating would also cause the customer more cost to purchase such a cable. Thus, the cable should be selected according the load current that calculated in section 4.3.2. Hence, all the cable is going to be installed 700mm underground for LV panel until distribution board and 800mm underground for the cable between TX and LV panel, the derating current should be calculated before select the cable.
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Cables Selection |
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Cable coordination |
Cable Specification |
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TX until LV panel |
Current (A) | Derating Current (A) | Cable Cross Section (mm2) |
Product Code |
|
|
Stadium LV panel |
868.12 | 1,296 | 800 |
CX1-T101-U80 |
|
|
Sport activity LV panel |
991.23 | 1,296 | 800 |
CX1-T101-U80 |
|
|
LV panel until distribution board |
|||||
|
Sport activity LV panel |
AC |
488.65 | 534 | 500 |
CX1-T104-W50 |
|
Socket + pump |
248 | 269 | 120 |
CX1-T104-W17 |
|
|
Lighting |
458.3 | 475 | 400 |
CX1-T104-W40 |
|
|
Firefighting |
270.8 | 296 | 150 |
CX1-T104-W18 |
|
|
Stadium LV panel |
AC + change room |
100.35 | 116 | 25 |
CX1-T104-W12 |
|
Audience + bathroom |
239.25 | 269 | 150 |
CX1-T104-W18 |
|
|
Poles (1+2) |
484.54 | 534 | 500 |
CX1-T104-W50 |
|
|
Poles (3+4) |
484.54 | 534 | 500 |
CX1-T104-W50 |
|
|
Parking + street |
379.4 | 384 | 240 |
CX1-T104-W20 |
|
|
Firefighting |
294.83 | 331 | 185 |
CX1-T104-W19 |
|